a) If is the radius of a circle then: | |

b) If is the radius of a circle then: | |

c) If is the radius of a circle then: | |

d) If and are radii of two concentric circles, then | |

e) If is an equilateral triangle of side .

Also if and are the radii of the circumscribed and inscribed circles of , then

f) Important notes:

- If two circles touch internally, then the distance between their centers is equal to the difference of their radii.
- If two circles touch externally. Then the distance between their centers is equal to the sum of their radii.
- The distance moved by a rotating wheel in one revolution is equal to the circumference of the wheel.
- The number of revolutions completed by a rotating wheel in one minute